In the field of printing, the demand is expected to grow for direct making of printing plates by digital recording. In order to meet this demand, methods have been proposed which comprise direct making of a printing plate by electrophotography, a silver salt diffusion transfer process, heat-sensitive digital recording using a thermal printer equipped with a thermal head or laser digital recording utilizing a technique for sensitizing photopolymer.
A known technique for direct making of a printing plate by a thermal head comprises transferring an image area onto a support having a hydrophilic layer with a thermal head by a melt transfer process to form a lipophilic image thereon, to thereby make a printing plate. However, it is difficult to place the printing plate prepared according to this method into practical use because it does not satisfy important requirements for a printing plate, such as good printability and good plate wear in printing machines.
It is well known that when irradiated with active light rays, an o-quinonediazide compound undergoes decomposition of a constituent diazo group and becomes a carboxyl-containing compound soluble in an alkali. Thus, when a photographic material comprising an o-quinonediazide compound is imagewise exposed to light and then developed with an alkaline developer, the exposed area is dissolved away with the developer while the unexposed area remains on the photographic material to form an image. This type of image recording material is called a positive-working image-forming material.
Furthermore, as described in JP-A-49-127615 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-56-6236, JP-A-56-36648, JP-B-62-49613 (The term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-62-49614, and JP-B-62-49615, a photographic material having a photosensitive layer comprising a composition containing an o-quinonediazide compound as a photosensitive component and an additive such as an aliphatic amine, a N-alcoholamine, an aromatic ketone, a quinone compound, an imidazole compound and a basic carbonium compound can also subjected to negative-working processing. In some detail, this type of photographic material can be heated at the same time as or after imagewise exposure, uniformly exposed to light, and then developed to form a negative image.
On the basis of these known techniques, an image-forming material was prepared comprising on a support a photosensitive layer containing an o-quinonediazide compound as a first essential component and an additive which directly or catalytically reacts with a photoreaction product of the o-quinonediazide compound on heating to produce an alkali-insoluble matter as a second essential component. The image-forming material thus prepared was exposed on the entire surface thereof to light rays which are active to the o-quinonediazide compound, typed with a thermal head printer, and then developed with an alkaline developer to remove the untyped areas. As a result, an image was formed, but the photosensitive layer caused contamination of the head. This contamination made it difficult to place the image-forming material into practical use.
Furthermore, the above noted additives substantially do not absorb light in the visible range. If a dye or dyestuff which absorbs in the visible range could be used as an additive for reversing (negative working), printing plates and various images could be formed by laser digital recording without masking (i.e., by uniformly exposing the image-forming material to light rays which are active to o-quinonediazide compounds (i.e., render the o-quinonediazide compound soluble in an alkaline developer), and then patternwise (i.e., imagewise) irradiating the material with a high output argon laser or the like so that the laser beam is converted to heat that renders insoluble those areas irradiated with the laser beam.
JP-B-62-49615 discloses a technique based on such a concept. In some detail, a photographic material comprising on a support a photosensitive composition containing an o-quinonediazide compound as a photosensitive component and a basic carbonyl dye selected from the group consisting of diphenylmethane dye, triphenylmethane dye, xanthene dye and acridine dye as an additive for reversal is disclosed. The photographic material is exposed on the entire surface thereof to light rays which are active to o-quinonediazide compounds, patternwise irradiated with a laser beam such as an argon laser, and then developed with an alkaline developer to dissolve away those areas that were not irradiated with the laser beam. Thus, a negative-working image is formed.
However, a system based on this technique has never been commercially available. This is largely because its low photosensitivity made it necessary to use a large-sized water-cooled laser or a very low scanning speed.
In recent years, on the other hand, semiconductor lasers emitting infrared laser beams have made remarkable Progress-miniaturization, higher output, multibeam laser composed of several to scores of these high output lasers. The progress in these techniques has made the foregoing system practicable if the photosensitivity of the image-forming material is in the range of from 100 mW to 400 mW.
The above cited JP-B-62-49615 describes that an infrared laser beam such as a YAG laser (oscillation wavelength: 1.06 .mu.m) can be used as well. However, basic carbonium dyes such as diphenylmethane dye, triphenylmethane dye, xanthene dye and acridine dye as reversing dyes for addition to the photosensitive composition include no coloring materials for converting light to heat which would be needed with use of an infrared laser. Thus, it is necessary to include an additive for converting light to heat in addition to the coloring material to effect reversal. This disadvantageously increases the amount of additives contained in the system, which results in deterioration of developability and image intensity.